Unsaturated ester molding compositions suffer from a variety of disadvantages depending on the particular composition and the particular desired end use. There has been much effort made toward minimizing or eliminating such disadvantages by modifying the basic compositions.
For example, U.S. Pat. No. 3,416,990, granted Dec. 17, 1968, to Robinson, states that the thermosetting character of polyester molding compositions presents certain manufacturing disadvantages and that attempts to overcome these with solid propylene polymers gives compositions with poor adhesion to glass. Robinson proposes to overcome this deficiency with compositions in which the propylene polymer has been partially modified by reaction with certain ethylenically unsaturated compounds.
U.S. Pat. No. 3,231,634, granted Jan. 25, 1966, to Wismer et al., states that polyester molding compositions which utilize unsaturated polyester frequently give finished products that are subject to "cracking or crazing or contain pits which render the casting porous or will not stand severe shock without breakage." Wismer discloses that these problems can be overcome or minimized by incorporating a small amount of polymer of a conjugated diene, for example, butadiene. It is now well recognized that inclusion of butadiene polymers into polyester molding compositions gives finished products with improved impact strength and possibly even reduced porosity, but such products generally lack the improved surface characteristics that are achieved with use of the composition of the present invention. Furthermore, although Wismer discloses (column 3, line 52) that fillers can be added, the clear focus of Wismer is toward unfilled compositions.
U.S. Pat. No. 3,701,748, granted Oct. 31, 1972, to Kroekel, states that polyester molding compositions frequently exhibit an unacceptable level of volume shrinkage when cured, and yield fibrous reinforced finished products with poor surface characteristics. Kroekel discloses that these problems can be overcome by including a thermoplastic polymer, for example, polymethacrylates, in the polyester composition.
U.S. Pat. No. 3,993,710, granted Nov. 23, 1976, to Alberts et al., notes similar problems to those observed by Kroekel and discloses using certain thermoplastic ethylene copolymers as an answer to these problems. Indeed, it is now well recognized that surface characteristics of products produced from polyester molding compositions can be greatly improved with thermoplastic polymers such as polyethylene and polymethacrylates; however, products made from such compositions generally lack high impact strength.
None of these techniques for improving conventional unsaturated polymerizable molding compositions has been able to achieve a finished product with both high impact strength and superior surface characteristics.